XXV111 THE EYE. 



seals at the Westminster Aquarium might have been frequently observed in 

 a dark corner of the building emitting a very suspicious-looking blue light. 

 This luminous appearance emanates from every part of the animal, whether 

 at the edges of the scales or on its internal surface when cut open, which, 

 if closely examined, is found to emit a slight moisture that can be scraped 

 off, diffused in water, and still remain luminous as observed among inver- 

 tebrates. 



VISION. 



Fishes, as a rule, possess two eyes, symmetrically placed one on either 

 side of the head, while they are undoubtedly large as compared with what 

 obtains in other vertebrates, or with the extent of surface in their own 

 bodies. Their size, however, is modified in accordance with local surround- 

 ings and their habits. Some frequent muddy waters, where vision would 

 be of less service than tactile organs ; in such cases appendages for feeling, 

 as barbels, are more developed than eyes. Others again are restricted to 

 living in dark caves where rays of light can hardly enter. Irrespective of 

 the foregoing cause of atrophy there are many forms residing in the dismal 

 abysses of the ocean where light is unable to penetrate, for solar i*ays are 

 computed to be lost at a depth of 200 fathoms or less, while in the fresh 

 waters of the Lake of Geneva, Professor Forel ascertained that at 30 

 fathoms photographic paper was entirely unacted upon. Another modifying 

 influence is whether the habits of the fish are nocturnal or diurnal, the 

 former, unless residing in dark places, requiring the larger organs of vision. 



Eyes likewise may be only rudimentally developed,' as in the Amphioxua 

 or the Myxine, wherein they appear as mere dark specks, but still have a 

 nerve distributed to them. Or they may have retrograded to so considerable 

 'an extent- as to be entirely absent in some forms which have taken up their 

 abodes in dark caverns, but even here the lens and other constituent 

 portions of the eye may usually be detected, whereas examples of the same 

 species living outside are provided with fully-formed eyes. The optic nerve 

 may degenerate, or disuse may occasion degeneration, but loss of light does 

 not necessarily end thus. 



The eyes of fishes are mostly situated in orbital cavities, where they rest 

 upon a cushion of adipose or gelatinous substance. Their direction is 

 subject to considerable variation, for although usually placed laterally, as 

 in the perch, salmon, or carp, they are not invariably so. The star-gazer, 

 Uranoscopus, has them on the upper surface of its head, where they are 

 situated somewhat close together. Many East Indian siluroids or sheat- 

 fishes, as Pseudeutropius or Ailia, have them even placed almost under- 

 neath and directed obliquely downwards and outwards. The sides of the 

 head may be laterally elongated, having these organs situated at their outer 



